The ability of a cell to maintain optimum protein synthetic activity impinges upon the ability to divide, to maintain homeostasis and metabolic integrity, and to repair damage. Protein synthetic activity depends, to a substantial extent, upon biosynthesis of ribosomes. A number of observations suggest that ribosome biosynthesis is impaired as a function of age. A series of experiments is proposed to confirm this hypothesis, to elucidate the mechanisms that account for decreased ribosome biosynthetic capacity in aged mice, and to determine the extent to which impairment of this process is related to failure to undergo a mitotic response. Ribosome biosynthetic capacity will be assessed by measurement of expression of component genes including those encoding 45S pre-rRNA (RDNA), 5S RNA, U3 small nuclear RNA (U3nRNA), and ribosomal protein S16 (rpS16). Techniques that are routine in this laboratory will be used to measure transcriptional activity in isolated nuclei, transcription of cloned genes in vitro, and abundance of the products. Experiments will be carried out to test the hypotheses (i) that transcription of these genes is coordinately diminished during aging and (ii) that decreased transcriptional activity is associated with a decrease in the amount or activity of trans-acting factors that are known to regulate the activities of these genes under other circumstances. These studies will be carried out in fractions derived from B lymphocytes of young and aged mice. Parallel studies will be carried out to test the hypothesis that failure B lymphocytes from aged mice to respond to mitotic stimulation relates to defective activation of ribosome biosynthesis (as determined by measurement of expression of the genes listed above).